The present invention relates to a light pipe adapted to form a reflection type liquid-crystal display device which is bright and easy to see, and a surface light source device which uses the light pipe and which is excellent in effective utilizing efficiency of light.
An illumination device permitting a reflection type liquid-crystal display device to be viewed in a dark place, or the like, has been required. In the meantime, the inventors of the present invention have tried to use a front-lighting system in which a back-lighting unit used in a transmission type liquid-crystal display device is disposed on the visual side of a liquid-crystal cell. Such a back-lighting unit uses a light pipe for outputting light incident on its side surface from one of its upper and lower surfaces through a light output means. In the front-lighting system, display contents are recognized visually through the light pipe.
In the back-lighting unit using the background-art light pipe, there was, however, a problem that it was difficult to put the back-lighting unit into practical use because of occurrence of shortage of contrast in the on-state of the back-lighting unit and occurrence of shortage of brightness, disorder of display, etc. in the off-state of the back-lighting unit. Incidentally, in a back-lighting unit using a light pipe having diffusing dots or fine irregularities as a light output means, there arose a problem that contrast was short in visual recognition in the on-state of the back-lighting unit whereas contrast and brightness of display were short as well as clarity was short because of remarkable disorder of a display image through the light pipe in visual recognition owing to external light such as room illumination, or the like, in the off-state of the back-lighting unit.
On the other hand, also in a back-lighting unit using a light pipe (JP-A-62-73206) having a stairstep prism structure including inclined surfaces at an inclination angle of 45degrees and flat surfaces at an inclination angle of 0 degrees as a light output means, there arose a problem that contrast and brightness of display were short in visual recognition in the on-state of the back-lighting unit.
Further, in the aforementioned front-lighting system, the light pipe was located as a surface on the visual side. When the surface was injured, the injury caused disorder of a display image in the same manner as in the fine irregularities. Particularly when the light output means in the light pipe was located in the surface and it was injured, leaking light was apt to increase or the light output means was apt to generate a bright or dark portion. Moreover, when the light output means was abraded gradually by wiping-out, or the like, there arose a problem that the light output characteristic of the light output means varied widely.
Therefore, an object of the present invention is to develop a reflection type liquid-crystal display device which is excellent in contrast and brightness of display in visual recognition regardless of on-state and off-state of the lighting unit, which is excellent in clarity because of prevention disorder of a display image through a light pipe and which is excellent in preservation of initial light output characteristic or display characteristic, and develop a light pipe and a surface light source device that can form the reflection type liquid-crystal display device.
According to the present invention, there is provided a light pipe in which: light incident on an incident side surface of said light pipe is outputted from a lower surface of said light pipe through a light output means formed on an upper surface of said light pipe; the direction of the largest intensity of the light outputted from said lower surface is within an angle of 30 degrees to a normal line with respect to a reference plane of said lower surface; the largest intensity of light leaking from said upper surface in the direction within said angle of 30 degrees is not higher than ⅕ as high as said largest intensity of the light outputted from said lower surface; and said light pipe has a hard coat layer on said upper surface so that light incident on said lower surface is transmitted out from said upper surface.
Further, according to the present invention, there is provided a surface light source device in which a light source is disposed on the incident side surface of the above-mentioned light pipe, and there is provided a reflection type liquid-crystal display device in which a liquid-crystal cell provided with a reflection layer is disposed on the lower surface side of the above-mentioned surface light source.
The operation and effect of the present invention are based on characteristic given to the light pipe. That is, while the inventors of the present invention have made eagerly investigation and investigation to overcome the aforementioned problems, the following fact has been found. In the background-art light pipe having diffusing dots or fine irregularities as a light output means, transmission light incident on the side surface is scattered by the light output means of the light pipe 18 so as to diverge in almost all directions as shown in FIGS. 9 and 10. The output light xcex11 from the lower surface and the leaking light xcex23 from the upper surface exhibit the largest strengths B and b in a direction xcex84 at an angle of about 60 degrees to a normal line (frontal direction) H with respect to the lower surface because of the scattering characteristic of the light output means. Because the strengths are approximately equal to each other, the quantity of light is short in a direction effective for visibility, especially in a viewing angle range of from upward 15 degrees to downward 30 degrees vertically and from leftward 30 degrees to rightward 30 degrees horizontally on the basis of the normal line. As a result, brightness of display is short in the viewing angle range. Further, because the output light xcex12 from the lower surface to form a display image overlaps the leaking light xcex24 from the upper surface, contrast is short. In the off-state of lighting, contrast is short because the display image is white-faded by the scattered light xcex32 and the display image is disordered remarkably because two kinds of display light xcex31 and xcex33 are mixed with each other by scattering owing to the light pipe.
The following fact has been further found. That is, also in the light pipe having the prism type light output means according to JP-A-62-73206, like the aforementioned light pipe, a large amount of light leaks from the upper surface. The leaking light overlaps the output light from the lower surface for forming the display image so that contrast is lowered. Moreover, a large amount of light outputs at a large output angle but the quantity of light in a direction effective for visibility is short to thereby lower brightness of display to cause a problem in lowering of display quality.
Therefore, in order to form a bright and clear display image, the light pipe needs to be provided as a light pipe capable of outputting light incident on the side surface from the lower surface with good directivity, especially with good light-condensing characteristic at an angle xcex83 as near to a direction of a normal line H with respect to the lower surface as possible as shown in FIG. 4 and particularly in the aforementioned viewing angle range. In a reflection type liquid-crystal display device, an attempt to achieve uniformity and clarity of display is generally made through a rough surface type reflection layer with a mean diffusion angle in a range of from about 5 to about 15 degrees. Hence, if the quantity of light incident on the reflection layer at a large angle to the reflection layer is large (B in FIG. 9, xcex11 in FIG. 10), the quantity of light effective for visibility is reduced to thereby make bright display difficult. Moreover, invisibility at such a large angle, reversion of display is apt to occur. In electric field birefringence type display, a problem in occurrence of a large color change etc. is apt to arise.
For improvement of contrast, the leaking light a from the upper surface needs to be prevented as sufficiently from overlapping the output light A from the lower surface to form a display image as possible, as shown in FIG. 4, especially needs to be prevented as sufficiently from overlapping the output light A in the aforementioned viewing angle range as possible. In the reflection type liquid-crystal display device, overlapping of the leaking light and the display image has large influence on the contrast ratio because the contrast ratio is generally in a range of from 1:5 to 1:20.
For prevention of disorder of the display image, light transmitted from the upper surface to the lower surface and from the lower surface to the upper surface needs to be prevented as sufficiently from being scattered as possible. A front-lighting unit provided in the reflection type liquid-crystal display device is an auxiliary light source for making visual recognition in a dark place possible. Hence, the front-lighting unit is adapted to visual recognition using external light such as indoor light or natural light for the original purpose of reduction of consumed electric power. Hence, if incidence of external light is blocked by the light pipe in the original condition that the front-lighting unit is switched off, display becomes dark. If the light is scattered by the light pipe in this condition, lowering of contrast is caused by white-fading of a surface or disorder of a display image is caused by mixture of the display image, or the like.
In addition, from the point of view of preservation of the aforementioned light output characteristic and display quality for a long term, the light pipe, especially the light output means formed on the upper surface of the light pipe, needs to be prevented from being injured and abraded by wiping-out, or the like. The pixel pitch of the liquid-crystal display device is generally in a range of from 100 to 300 xcexcm. Hence, the influence of the light pipe on transmitted light needs to be suppressed as sufficiently as possible so that information with a pitch of about 100 xcexcm can be recognized visually clearly. At the same time, moire caused by interference with pixels needs to be also suppressed to obtain good display quality.